1. Field of the Invention
The present invention relates to the field of electrical drives. Specifically, it relates to a method for open-loop and closed-loop control of an electrical drive which comprises an asynchronous motor which is supplied by a DC-AC invertor with a downstream sine-wave output filter with an AC voltage at a predetermined circular frequency.
2. Discussion of Background
Drives having asynchronous motors in which the AC voltage that is required for the drive and is at a selectable frequency is produced by a DC-AC invertor from a DC voltage intermediate circuit or some other DC voltage source by appropriate actuation of power semiconductors which are arranged in a bridge circuit, are frequently equipped at the output of the invertor with LC filters which act as sine-wave output filters and convert the steep-edged voltage pulses from the invertor into a sinusoidal voltage with a variable frequency and amplitude for the motor.
The motor insulation and the bearings of the asynchronous motor, which are frequently subject to problems resulting from the steep edges when pulsed invertors are used are far less at risk owing to the filtered sinusoidal voltage. Furthermore, a sinusoidal motor voltage reduces the iron losses, the audible noise in the machine, and the electromagnetic interference fields.
If a drive is equipped with a sine-wave output filter, natural oscillations of the filter also have to be taken into account. The invertor itself must be operated such that these oscillations are damped, since it is normally impractical to rely on the natural damping of the filter. Furthermore, the interactions between the asynchronous motor and the filter also have to be considered. Since the asynchronous motor is a non-linear load, these interactions are extremely complex. Any closed-loop control system used for the drive should take account of these problems.
In order that the combination of the invertor, filter and asynchronous machine operates in a stable manner and at the same time has a rapid dynamic response, a standard solution shall be found for the operation of the combination. At the same time, it is desirable to minimize the complexity in terms of measurement devices and measurements required for the closed-loop control system.
Known closed-loop control methods generally use one of the following two solutions:
At least two sensors are used to measure the current in the filter capacitors of the output filter. However, this leads to additional costs and necessitates additional space in the arrangement. PA1 A high-pass filter is used in the closed-loop control system for the output voltage. This has the disadvantage that the regulator response is very highly dependent on the output frequency, and that there is a frequency-dependent difference, which is stable with respect to time, between the required value and the actual value of the output voltage, which difference has a disadvantageous effect for high-power asynchronous drives. Furthermore, the high-pass filter--in the same way as a differential regulator--is sensitive to measurement noise.